C++11 Print run-time variable name equal to some constexpr variable

Question

Is there any way, using c++11, to get the name of a variable that is a (possibly static) constexpr.

For example:

struct Foo{
    int x, y, z;
};

constexpr Foo PrintMe = { 1, 2, 3};

I would like to get a string "PrintMe" somehow.

I know that I can use a macro like:

#define NAME_OF( v ) #v

and call

std::cout << NAME_OF(PrintMe) << std::endl;

which will print

PrintMe

Is there a way to get the following print the same?

Foo a = PrintMe;

std::cout << Magic(a) << std::endl;

EDIT: I am not looking for some magic solution which will make the call to Magic(a) work. I understand that doing something to accommodate what I want will require defining some macros or templates. Like enums can be printed in some sort of way (How to convert an enum type variable to a string?

What you are looking for is called reflection and it is not part of C++. — NathanOliver, Feb 14 '17 at 16:00
No. BTW what it has anything to do with `constexpr`. It is not possible in C++, irrespective of whether it is `constexpr` or not. — Nawaz, Feb 14 '17 at 16:01
Also your title is misleading. You have a way to print a compile time variable, you are looking to print a run time variable. — NathanOliver, Feb 14 '17 at 16:01
The same could be said on any "How to print the name of an enum" but there are way of creating those enums in such a way that their names could be printed — ZivS, Feb 14 '17 at 16:05
@NathanOliver I changed the title, to better fit the question — ZivS, Feb 14 '17 at 16:06
_"there are way of creating those enums in such a way that their names could be printed"_ Mmmm not really. Only to create a separate lookup. You can make that more convenient with macros, which is the same thing being claimed here. — Lightness Races in Orbit, Feb 14 '17 at 16:11

score 2 · Answer 1 · answered Feb 14 '17 at 16:01

2

If you want to do this without macro, there is no way, no. You would have to do some kind of meta class in order to achieve this.

answered Feb 14 '17 at 16:01

Telokis

3,399
14
36

It doesn't exist yet, see http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0194r3.html for a proposal. Until then you need something outside the language that processes your source to generate extra information (such as Qt's `moc`) – Jonathan Wakely Feb 14 '17 at 16:15

score 0 · Answer 2 · answered Feb 14 '17 at 16:07

Foo a = PrintMe; has value semantics, it assigns the value of PrintMe to a. Afterwards, there is no way of checking whether this value came from PrintMe. Of course, you can check for the same value:

std::string Magic(const Foo& a) {
    if (a == PrintMe) { return NAME_OF(PrintMe); };
    else { /* whatever you want to happen here */ }
}

But this will of course also return "PrintMe" if you did

Foo a = {1, 2, 3};

Because the value is the same as PrintMe.

C++11 Print run-time variable name equal to some constexpr variable

2 Answers2